The present invention relates to an oil ring.
Conventionally, oil rings attached to pistons of engines are known. For example, JP2007-504416A discloses an oil ring including an expansion spring (expander) and a pair of annular disks (side rails). An outer circumferential surface of each disk is formed to bulge outward in radial directions of the disk. The outer circumferential surface slides on a cylinder inner circumferential surface with an oil film intervening therebetween. A tip portion (portion protruding the most radially outward) of the outer circumferential surface is offset from the center of the disk in its thickness direction (i.e., axial direction). In other words, the outer circumferential surface has a shape which is asymmetric with respect to a plane passing through the center of the disk in the thickness direction perpendicular to the thickness direction.
Incidentally, the outer circumferential surface of each disk of JP2007-504416A is formed by two different curved surfaces on the combustion chamber side and the crank chamber side with respect to the tip portion. Specifically, when the outer circumferential surface is cut in a plane perpendicular to a circumferential direction of the disk, the cross section of the curved surface on the combustion chamber side and the cross section of the curved surface on the crank chamber side are formed by two different quadratic curves. Further, the curved surfaces are coupled to each other at the tip portion, and the connecting portion, i.e., a tip portion, has an edge shape, i.e., is edged.
If the tip portion is edged, the pressure on the tip portion becomes excessively high, and the tip portion may be abraded. Especially when the sliding speed of the outer circumferential surface is zero, such as when the piston is at the top dead center or the bottom dead center, the tip portion may contact with the cylinder inner circumferential surface, which causes abrasion of the cylinder inner circumferential surface as well. If the tip portion or the cylinder inner circumferential surface is abraded, the oil consumption increases. Moreover, if the oil film is extremely thin or any part of the disk contacts with the cylinder inner circumferential surface, the sliding resistance may increase.
Further, near the tip portion of the outer circumferential surface of each disk of JP2007-504416A, the curved surface on the combustion chamber side has a larger inclination than that of the curved surface on the crank chamber side with respect to the cylinder inner circumferential surface. In general, oil rings are required to have a function to form an oil film with suitable thickness when the piston moves to the combustion chamber side, while scraping off unnecessary oil when the piston moves to the crank chamber side. However, in the disk of JP2007-504416A, the curved surface on the combustion chamber side near the tip portion has the larger inclination with respect to the cylinder inner circumferential surface, and thus, the oil film with the suitable thickness cannot be formed and the sliding resistance may increase.